The present disclosure generally relates to methods for producing and purifying 2-aryl-3,3-bis(4-hydroxyaryl)phthalimidine compounds, the purified compounds, and polycarbonates as well as other polymers derived from the compounds.
3,3-Bis-(4-hydroxy-phenyl)-3H-isobenzofuran-1-one (hereinafter referred to as phenolphthalein) has been used as an aromatic dihydroxy monomer for preparing polycarbonates, which are generally characterized with excellent ductility and high glass transition temperatures. Certain derivatives of phenolphthalein have also been used as aromatic dihydroxy monomers to prepare polycarbonates. For example, polycarbonate homopolymers have been prepared by an interfacial polycondensation method using phosgene and phenolphthalein derivatives such as 3,3-bis(4-hydroxyphenyl)phthalimidine and 2-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine (“PPPBP”).
Lin and Pearce (Journal of Polymer Science: Polymer Chemistry Edition, (1981) Vol. 19, pp. 2659-2670) reported the synthesis of PPPBP by refluxing phenolphthalein and aniline hydrochloride in aniline for 6 hours, followed by recrystallization from ethanol. During this reaction, side products are created which, if not removed, can result in the PPPBP having an unacceptable purity for use as a monomer or as a comonomer in subsequent polymerization reactions. The impurities in the PPPBP include, for example, levels of phenolphthalein or phenolphthalein compounds that can undesirably produce discoloration in the polycarbonates and other polymers derived therefrom. Coloration is not a desirable attribute for many commercial applications. U.S. Pat. No. 5,344,910 discloses that copolymers of PPPBP were found to have poor melt stability resulting in foamy polymer melts and moldings, and discoloration of the polymer during melt processing.
U.S. Publication 2005/0288517 describes a method of purifying 2-aryl-3,3-bis(4-hydroxyaryl)phthalimidine compounds. The technique described in U.S. Publication No. 2005/0288517 achieved phenolphthalein-based impurity levels as low as about 300 ppm based on PPPBP weight. While these levels improved the color and stability of polymers prepared with the purified PPPBP, further improvements in color and stability of polymer made with PPPBP are desirable.
It would therefore be desirable to develop a process for further improving the purity of phenolphthalein derivatives such as 2-aryl-3,3-bis(4-hydroxyaryl)phthalimidine and PPPBP.